This class provides a skeletal implementation of the Map
interface, to minimize the effort required to implement this interface.

To implement an unmodifiable map, the programmer needs only to extend this
class and provide an implementation for the entrySet method, which
returns a set-view of the map's mappings. Typically, the returned set
will, in turn, be implemented atop AbstractSet. This set should
not support the add or remove methods, and its iterator
should not support the remove method.

To implement a modifiable map, the programmer must additionally override
this class's put method (which otherwise throws an
UnsupportedOperationException), and the iterator returned by
entrySet().iterator() must additionally implement its
remove method.

The programmer should generally provide a void (no argument) and map
constructor, as per the recommendation in the Map interface
specification.

The documentation for each non-abstract method in this class describes its
implementation in detail. Each of these methods may be overridden if the
map being implemented admits a more efficient implementation.

containsValue

Returns true if this map maps one or more keys to the
specified value. More formally, returns true if and only if
this map contains at least one mapping to a value v such that
(value==null ? v==null : value.equals(v)). This operation
will probably require time linear in the map size for most
implementations of the Map interface.

This implementation iterates over entrySet() searching
for an entry with the specified value. If such an entry is found,
true is returned. If the iteration terminates without
finding such an entry, false is returned. Note that this
implementation requires linear time in the size of the map.

containsKey

Returns true if this map contains a mapping for the specified
key. More formally, returns true if and only if
this map contains a mapping for a key k such that
(key==null ? k==null : key.equals(k)). (There can be
at most one such mapping.)

This implementation iterates over entrySet() searching
for an entry with the specified key. If such an entry is found,
true is returned. If the iteration terminates without
finding such an entry, false is returned. Note that this
implementation requires linear time in the size of the map; many
implementations will override this method.

get

Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key
k to a value v such that (key==null ? k==null :
key.equals(k)), then this method returns v; otherwise
it returns null. (There can be at most one such mapping.)

If this map permits null values, then a return value of
null does not necessarily indicate that the map
contains no mapping for the key; it's also possible that the map
explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

This implementation iterates over entrySet() searching
for an entry with the specified key. If such an entry is found,
the entry's value is returned. If the iteration terminates without
finding such an entry, null is returned. Note that this
implementation requires linear time in the size of the map; many
implementations will override this method.

Parameters:

key - the key whose associated value is to be returned

Returns:

the value to which the specified key is mapped, or
null if this map contains no mapping for the key

put

Associates the specified value with the specified key in this map
(optional operation). If the map previously contained a mapping for
the key, the old value is replaced by the specified value. (A map
m is said to contain a mapping for a key k if and only
if m.containsKey(k) would return
true.)

the previous value associated with key, or
null if there was no mapping for key.
(A null return can also indicate that the map
previously associated null with key,
if the implementation supports null values.)

remove

Removes the mapping for a key from this map if it is present
(optional operation). More formally, if this map contains a mapping
from key k to value v such that
(key==null ? k==null : key.equals(k)), that mapping
is removed. (The map can contain at most one such mapping.)

Returns the value to which this map previously associated the key,
or null if the map contained no mapping for the key.

If this map permits null values, then a return value of
null does not necessarily indicate that the map
contained no mapping for the key; it's also possible that the map
explicitly mapped the key to null.

The map will not contain a mapping for the specified key once the
call returns.

This implementation iterates over entrySet() searching for an
entry with the specified key. If such an entry is found, its value is
obtained with its getValue operation, the entry is removed
from the collection (and the backing map) with the iterator's
remove operation, and the saved value is returned. If the
iteration terminates without finding such an entry, null is
returned. Note that this implementation requires linear time in the
size of the map; many implementations will override this method.

Note that this implementation throws an
UnsupportedOperationException if the entrySet
iterator does not support the remove method and this map
contains a mapping for the specified key.

Parameters:

key - key whose mapping is to be removed from the map

Returns:

the previous value associated with key, or
null if there was no mapping for key.

putAll

Copies all of the mappings from the specified map to this map
(optional operation). The effect of this call is equivalent to that
of calling put(k, v) on this map once
for each mapping from key k to value v in the
specified map. The behavior of this operation is undefined if the
specified map is modified while the operation is in progress.

keySet

Returns a Set view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation), the results of
the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations. It does not support the add or addAll
operations.

This implementation returns a set that subclasses AbstractSet.
The subclass's iterator method returns a "wrapper object" over this
map's entrySet() iterator. The size method
delegates to this map's size method and the
contains method delegates to this map's
containsKey method.

The set is created the first time this method is called,
and returned in response to all subsequent calls. No synchronization
is performed, so there is a slight chance that multiple calls to this
method will not all return the same set.

Returns:

a set view of the keys contained in this map

values

Returns a Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own remove operation),
the results of the iteration are undefined. The collection
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll and clear operations. It does not
support the add or addAll operations.

This implementation returns a collection that subclasses AbstractCollection. The subclass's iterator method returns a
"wrapper object" over this map's entrySet() iterator.
The size method delegates to this map's size
method and the contains method delegates to this map's
containsValue method.

The collection is created the first time this method is called, and
returned in response to all subsequent calls. No synchronization is
performed, so there is a slight chance that multiple calls to this
method will not all return the same collection.

Returns:

a collection view of the values contained in this map

entrySet

Returns a Set view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation, or through the
setValue operation on a map entry returned by the
iterator) the results of the iteration are undefined. The set
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll and
clear operations. It does not support the
add or addAll operations.

equals

Compares the specified object with this map for equality. Returns
true if the given object is also a map and the two maps
represent the same mappings. More formally, two maps m1 and
m2 represent the same mappings if
m1.entrySet().equals(m2.entrySet()). This ensures that the
equals method works properly across different implementations
of the Map interface.

This implementation first checks if the specified object is this map;
if so it returns true. Then, it checks if the specified
object is a map whose size is identical to the size of this map; if
not, it returns false. If so, it iterates over this map's
entrySet collection, and checks that the specified map
contains each mapping that this map contains. If the specified map
fails to contain such a mapping, false is returned. If the
iteration completes, true is returned.

hashCode

public int hashCode()

Returns the hash code value for this map. The hash code of a map is
defined to be the sum of the hash codes of each entry in the map's
entrySet() view. This ensures that m1.equals(m2)
implies that m1.hashCode()==m2.hashCode() for any two maps
m1 and m2, as required by the general contract of
Object.hashCode().

toString

Returns a string representation of this map. The string representation
consists of a list of key-value mappings in the order returned by the
map's entrySet view's iterator, enclosed in braces
("{}"). Adjacent mappings are separated by the characters
", " (comma and space). Each key-value mapping is rendered as
the key followed by an equals sign ("=") followed by the
associated value. Keys and values are converted to strings as by
String.valueOf(Object).

CloneNotSupportedException - if the object's class does not
support the Cloneable interface. Subclasses
that override the clone method can also
throw this exception to indicate that an instance cannot
be cloned.